Group iii-n nanowire transistors

ABSTRACT

A group III-N nanowire is disposed on a substrate. A longitudinal length of the nanowire is defined into a channel region of a first group III-N material, a source region electrically coupled with a first end of the channel region, and a drain region electrically coupled with a second end of the channel region. A second group III-N material on the first group III-N material serves as a charge inducing layer, and/or barrier layer on surfaces of nanowire. A gate insulator and/or gate conductor coaxially wraps completely around the nanowire within the channel region. Drain and source contacts may similarly coaxially wrap completely around the drain and source regions.

This is a Continuation of application Ser. No. 14/936,609 filed Nov. 9,2015 which is Divisional of application Ser. No. 13/976,413 filed Jun.26, 2013 now U.S. Pat. No. 9,240,410 issued Jan. 19, 2016 which is aU.S. National Phase Application under 35 U.S.C. 317 of InternationalApplication No. PCT/US2011/065919 filed Dec. 19, 2011.

TECHNICAL FIELD

Embodiments of the present invention generally relate to microelectronicdevices and fabrication, and more particularly to group III-Ntransistors.

BACKGROUND

Systems on a chip (SoC) have been implemented in a number of capacitiesover the last few decades. SoC solutions offer the advantage of scalingwhich cannot be matched by board-level component integration. Whileanalog and digital circuits have long been integrated onto a samesubstrate to provide a form of SoC that provides mixed signalcapabilities, SoC solutions for mobile computing platforms, such assmart phones and tablets, remain elusive because these devices typicallyinclude components which operate with two or more of high voltage, highpower, and high frequency. As such, conventional mobile computingplatforms typically utilize group III-V compound semiconductors, such aGaAs heterojunction bipolar transistors (HBTs), to generate sufficientpower amplification at GHz carrier frequencies, and laterally diffusedsilicon MOS (LDMOS) technology to manage voltage conversion and powerdistribution (battery voltage regulation including step-up and/orstep-down voltage conversion, etc.). Conventional silicon field effecttransistors implementing CMOS technology is then a third devicetechnology utilized for logic and control functions within a mobilecomputing platform.

The plurality of transistor technologies utilized in a mobile computingplatform limits scalability of the device as a whole and is therefore abarrier to greater functionality, higher levels of integration, lowercosts, and smaller form factors, etc. While an SoC solution for themobile computing space that would integrate two or more of these threedevice technologies is therefore attractive, one barrier to an SoCsolution is the lack of a scalable transistor technology having bothsufficient speed (i.e., sufficiently high gain cutoff frequency, F_(t)),and sufficiently high breakdown voltage (BV).

One promising transistor technology is based on group III-nitrides(III-N). However, this transistor technology faces fundamentaldifficulties in scaling to feature sizes (e.g., gate length) less than100 nm where short channel effects become difficult to control. ScaledIII-N transistors with well-controlled short channel effects aretherefore important to achieving high F_(t), with sufficiently highbreakdown voltage (BV). For an SoC solution to deliver the productspecific electrical current and power requirements of a mobile computingplatform, fast switching high voltage transistors capable of handlinghigh input voltage swings and providing high power added efficiencies atRF frequencies are needed. An advanced III-N transistor amenable toscaling and such performance is therefore advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not by way of limitation, and can be more fully understood withreference to the following detailed description when considered inconnection with the figures, in which:

FIG. 1A is an isometric illustration of a group III-N transistor, inaccordance with an embodiment;

FIG. 1B is a cross-sectional view of a channel region of the group III-Ntransistor illustrated in FIG. 1A;

FIG. 1C is an isometric illustration of a group III-N transistor, inaccordance with an embodiment;

FIG. 2A is a isometric illustration of a GaN crystal orientation for agroup III-N transistor, in accordance with an embodiment;

FIG. 2B is a cross-sectional view of a channel region of a group III-Ntransistor employing a non-planar GaN body having the crystalorientation illustrated in FIG. 2A;

FIG. 2C is a band diagram for the channel region illustrated in FIG. 2B;

FIG. 2D is a isometric illustration of a GaN crystal orientation for agroup III-N transistor, in accordance with an embodiment;

FIG. 2E is a cross-sectional view of a channel region of a group III-Ntransistor employing a non-planar GaN body having the crystalorientation illustrated in FIG. 2D;

FIG. 3 is a flow diagram illustrating a method of fabricating anon-planar high voltage transistor, in accordance with an embodiment;

FIGS. 4A, 4B, 4C, 4D and 4E are isometric illustrations of a non-planarhigh voltage transistor fabricated in accordance with an embodiment ofthe method illustrated in FIG. 3; and

FIG. 5 is a functional block diagram of an SoC implementation of amobile computing platform, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

In the following description, numerous details are set forth, however,it will be apparent to one skilled in the art, that the presentinvention may be practiced without these specific details. In someinstances, well-known methods and devices are shown in block diagramform, rather than in detail, to avoid obscuring the present invention.Reference throughout this specification to “an embodiment” means that aparticular feature, structure, function, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrase “in an embodiment” invarious places throughout this specification are not necessarilyreferring to the same embodiment of the invention. Furthermore, theparticular features, structures, functions, or characteristics may becombined in any suitable manner in one or more embodiments. For example,a first embodiment may be combined with a second embodiment anywhere thetwo embodiments are not mutually exclusive.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe structural relationships between components.It should be understood that these terms are not intended as synonymsfor each other. Rather, in particular embodiments, “connected” may beused to indicate that two or more elements are in direct physical orelectrical contact with each other. “Coupled” my be used to indicatedthat two or more elements are in either direct or indirect (with otherintervening elements between them) physical or electrical contact witheach other, and/or that the two or more elements co-operate or interactwith each other (e.g., as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one material layer with respect to other layers. Assuch, for example, one layer disposed over or under another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. Moreover, one layer disposed between two layers maybe directly in contact with the two layers or may have one or moreintervening layers. In contrast, a first layer “on” a second layer is indirect contact with that second layer.

Described herein are embodiments of group III-nitride (III-N)semiconductor nanowires and fabrication techniques to enable thefabrication of high voltage, high bandwidth field effect transistors. Inparticular embodiments, such transistors are employed in SoCarchitectures integrating high power wireless data transmission and/orhigh voltage power management functions with low power CMOS logic dataprocessing. The nanowire structure entails a “gate-all-around” forexcellent electrostatic control of short-channel effects, and thus,permit ultra-scaling of group III-N transistors into the sub-100 nmregime. High frequency operation suitable for broadband wireless datatransmission applications is possible while the use of large bandgapIII-N materials also provides a high BV such that sufficient RF outputpower may be generated for the wireless data transmission applications.This combination of high F, and high voltage capability also makespossible the use of the transistors described herein for high speedswitching applications in DC-to-DC converters utilizing inductiveelements of reduced size. As both the power amplification and DC-to-DCswitching applications are key functional blocks in smartphones,tablets, and other mobile platforms, the structures described herein maybe utilized in a SoC solution for such devices.

In embodiments, a multi-layered semiconductor structure, which mayinclude a plurality of group III-N materials is leveraged to form ananowire, a plurality of vertically stacked nanowires, and may befurther utilized to incorporate a semiconductor materials having adiffering bandgap into various regions of the transistor (e.g., a widerbandgap material may be incorporated in an extrinsic drain regionbetween the device channel and the drain contact). In the exemplaryembodiments, gate structures wrap around all sides of the channel regionto provide fully gated confinement of channel charge for scaling of gatelengths (L_(g)). Depending on the embodiment, one or more surfaces ofthe nanowire are covered with a wide bandgap group III-N material toprovide one or more of: enhanced channel mobility, sheet charge [twodimensional electron gas (2DEG)] by spontaneous and piezoelectricpolarization, passivation of interfacial states, and energy barriers forchannel charge carrier confinement.

FIG. 1A is an isometric illustration of a group III-N transistor 100, inaccordance with an embodiment. Generally, the group III-N transistor 100is a gate voltage controlled device (i.e., a FET) commonly referred toas a high electron mobility transistor (HEMT). The group III-Ntransistor 100 includes at least one non-planar crystallinesemiconductor body that is on a plane parallel to a top surface of asubstrate layer 205 upon which the transistor 100 is disposed. In anembodiment, the substrate layer 205 is insulating or semi-insulatingand/or has an insulating or semi-insulating layer disposed there on,over which the nanowire 210A is disposed. In one such embodiment, thesubstrate layer 205 is a top layer of group III-N semiconductor grown(depicted in FIG. 1A) on a support substrate or transferred onto a donorsubstrate (support and donor substrates not depicted). In a particularembodiment, the substrate layer 205 includes a silicon supportsubstrate, upon which the group III-N layer is epitaxially grown,however, the support substrate may also be of alternate materials, whichmay or may not be combined with silicon, including, but not limited to,germanium, indium antimonide, lead telluride, indium arsenide, indiumphosphide, gallium arsenide, or gallium antimonide, carbon (SiC), andsapphire. In another embodiment, the substrate layer 205 upon which thetransistor 100 is disposed is a dielectric layer, so that the substratelayer 205 is a buried oxide (BoX) which may be formed for example bytransferring one or more layers of the semiconductor from which thenanowire 210A is formed onto the substrate layer 205.

As further illustrated in FIG. 1A, a longitudinal length L of the groupIII-N transistor 100 is divided between a source region 220, a channelregion 145, a drain region 230, and an extrinsic drain region 235Adisposed there between. The extrinsic drain region 235A is specificallyhighlighted as it has a significant effect on BV of the device. Withinat least the channel region 245, the non-planar crystallinesemiconductor body is physically separated from the substrate layer 205by an intervening material other than the crystalline semiconductorforming the body or the material forming the substrate layer 205 to forma laterally oriented nanowire 210A. For the embodiments describedherein, the transverse cross-sectional geometry of the nanowire 210A mayvary considerably from circular to rectangular such that the thicknessof the nanowire 210A (i.e., in z dimension) may be approximately equalto a width of the nanowire 210A (i.e., in y dimension) or the thicknessand width of the nanowire 210A may be significantly different from eachother (i.e., physically akin to a ribbon, etc.) to form cylindrical andparallelepiped semiconductor bodies. For the exemplary embodiments, thewidth of the nanowire 210A is between 5 and 50 nanometers (nm), but thismay vary depending on implementation.

Within the channel region 245, the nanowire 210A has much greater longrange order than a polycrystalline material. In the exemplaryembodiment, the channel region 245 is substantially single crystallineand although is referred to herein as “monocrystalline,” one of ordinaryskill will appreciate that a low level of crystal defects maynevertheless be present as artifacts of an imperfect epitaxial growthprocess. Within the channel region 245, the nanowire 210A is acrystalline arrangement of a first semiconductor material including oneor more group III elements and nitrogen (i.e., a group III-Nsemiconductor). Generally, this first group III-nitride material in thechannel region 245 should have relatively high carrier mobility andtherefore in embodiments, the channel region 245 is to be asubstantially undoped group III-nitride material (i.e., impurityconcentration minimized) for minimal impurity scattering. In a firstexemplary embodiment, the channel region 245 is GaN. In a secondexemplary embodiment, the channel region 245 is indium nitride (InN).Because InN has a greater carrier mobility the GaN (2700 cm²/Vs ascompared to 1900 cm²/Vs), transistor metrics, such as specific onresistance (R_(on)), may be relatively better for InN embodiments. In athird exemplary embodiment, the channel region 245 is a ternary alloy ofGaN, such as aluminum gallium nitride (Al_(x)Ga_(1-x)N), where x is lessthan 1. In a fourth exemplary embodiment, the channel region 245 is aternary alloy of InN, such as aluminum indium nitride (Al_(x)In_(1-x)N),where x is less than 1. In further embodiments, the channel region 245is a quaternary alloy including at least one group III element andnitrogen, such as In_(x)Al_(y)Ga_(1-x-y)N.

Within at least the channel region 245, the nanowire 210A is coveredwith a second semiconductor layer 215 disposed over one or more of thesidewalls, top, and/or bottom surfaces of the nanowire 210A. In theexemplary embodiment, the semiconductor layer 215 is disposed directlyat least two opposite surfaces of the nanowire 210A. FIG. 1B is across-sectional view along the B plane through the channel region 245 ofthe group III-N transistor 200 illustrating the semiconductor layer 215as disposed on all perimeter surfaces of nanowire 210A (and nanowire210B). As further illustrated in FIG. 2A, the semiconductor layer 215also covers the nanowire 210A within the extrinsic drain region 235Awhere the layer 215 functions as a charge inducing layer.

The semiconductor layer 215 is of a second semiconductor compositionhaving a wider bandgap than that of the group III-N material utilized inthe nanowire 210A within the channel region 245. Preferably, thecrystalline semiconductor layer 215 is substantially monocrystalline(i.e., having a thickness below the critical thickness), lattice matchedto the group III-N material utilized in the nanowire 210A within thechannel region 245. In the exemplary embodiment, the semiconductor layer215 is of a second group III-N material having the same crystallinity asthat of the nanowire 210 to form a quantum-well heterointerface withinthe channel region 245.

Generally, any of the group III-N materials described for the nanowire210A may be utilized for the semiconductor layer 215, as dependent onthe material selected for the nanowire 210A to provide the semiconductorlayer 215 with a larger bandgap than the nanowire 210A. In a firstexemplary embodiment where the nanowire 210A is GaN, the semiconductorlayer 215 is AlN. In a second exemplary embodiment where the nanowire210A is InN, the semiconductor layer 215 is GaN. In a third embodimentwhere the nanowire 210A is Al_(x)In_(1-x)N, the semiconductor layer 215is Al_(y)In_(1-y)N, where y is greater than x. In a fourth embodimentwhere the nanowire 210A is Al_(y)Ga_(1-x)N, the semiconductor layer 215is Al_(y)Ga_(1-y)N, where y is greater than x. Quaternary alloysincluding at least one group III element and nitrogen, such asAl_(1-x-y)In_(x)Ga_(y)N (x,y<1), are also possible. The semiconductorlayer 215 may further comprise any multilayer stack of these GroupIII-Nitrides, for example, an Al_(x)In_(1-x)N/AlN stack with the AlNlayer of the stack adjacent to the (e.g., GaN) nanowire 210A.

Even where the semiconductor layer 215 is present on all surfaces of thenanowire 210A (top, bottom, and sidewalls), as depicted in FIG. 1B,though thickness may vary. The semiconductor layer 215 serves at leasttwo functions within the channel region 245. The two functions are aresult of the asymmetry induced by the polar bonds within thesemiconductor crystalline body 210A and the polarization directionstemming from these bonds with respect to the semiconductor layer 215.Depending on the crystal orientation of the nanowire 210A, variousopposing surfaces of the nanowire 210A are aligned with differentcrystal planes so that the effect of an electric field resulting fromthe semiconductor layer 215 on these opposing surfaces varies.

In one embodiment, illustrated by FIG. 2A, the group III-nitride of thenanowire 210A has the crystal structure is known as the wurtzitestructure. The GaN and other group III-nitrides described herein may beformed with the wurtzite structure which is notable in that it isnon-centrosymmetric meaning that the crystal lacks inversion symmetry,and more particularly the {0001} planes are not equivalent. For theexemplary GaN embodiment, one of the {0001} planes is typically referredto as the Ga-face (+c polarity) when and the other referred to as theN-face (−c polarity). Often for planar group III-N devices, one or theother of {0001} planes is more proximate a substrate surface and so maybe referred to as Ga polarity (+c) if the three bonds of the Ga (orother group III element) point towards the substrate or N polarity (−c)if the three bonds of the Ga (or other group III element) point awayfrom the substrate. For a first embodiment of the group III-N transistor200 however, the wurtzite crystal orientation is such that the (1010)plane having lattice constant c forms a top surface of the crystal andinterfaces the substrate layer 205.

As further illustrated in FIG. 2B, where the nanowire 210A has the groupIII-nitride material oriented as in FIG. 2A, the semiconductor layer 215includes sidewall portions 215A and 215B which function as a chargeinducing layer on one of the sidewalls (e.g., 215B) such that a 2DEG211A may be formed in the nanowire 210A adjacent to the semiconductorlayer 215. As so oriented, the semiconductor layer sidewall portion 215Amay further serve as a back barrier to confine charge carriers withinthe channel region 245. As shown, the first sidewall portion 215A issubstantially along the (0001) plane such that the majority of thesurface of the first sidewall portion 215A is on the (0001) plane of thenanowire 210A. Similarly, the second sidewall portion 215B issubstantially along the (0001) plane such that the majority of thesurface of the second sidewall 110B is defined by the (0001) plane. Thespontaneous polarization field, P_(SP) within the nanowire 210A istherefore directed from the second sidewall portion 215B to the firstsidewall portion 215B. As such, the polarization of the non-planar groupHI-N transistor 200 is through the width of the nanowire 210A (laterallyalong the y-dimension).

As further illustrated in FIG. 2B, because of the crystallinity of thesemiconductor layer 215 a spontaneous polarization field, P_(SP), isalso present within the semiconductor layer 215 and is aligned with thatof the nanowire 210A. Furthermore, when the semiconductor layer 215 isunder tensile strain in a direction parallel to the height dimension ofthe sidewall portions 215A and 215B (along z-dimension as shown in FIG.2B), a piezoelectric polarization field P_(PE) is also aligned withP_(SP) in a direction from the second sidewall portion 215B to the firstsidewall portion 215A. The polarizations of the nanowire 210A and thesemiconductor layer 215 therefore form a polarization field at theheterointerface along the (0001) plane proximate to the second sidewallportion 215B. As described by Maxwell's equations, the polarizationfield induces the 2DEG 211A.

For a second embodiment of the group III-N transistor 200, the wurtzitecrystal orientation is such that the (0001) plane having latticeconstant a forms a top surface of the crystal and interfaces thesubstrate layer 205, as illustrated in FIG. 2D. For this embodiment, asfurther illustrated in FIG. 4E, the semiconductor layer 215 present onthe top and bottom of the nanowire 210A (215D and 215C) functions ascharge inducing layer and back barrier, respectively. The semiconductorlayer 215 spontaneous polarization field, P_(SP) within the nanowire210A is then directed from the top surface portion 215D to the bottomsurface portion 215C. As such, the polarization of the non-planar groupIII-N transistor 200 is through the thickness of the nanowire 210A(vertically along the z-dimension). As illustrated in FIG. 2E, thepolarizations of the nanowire 210A and the semiconductor layer portions215D and 215C therefore form polarization fields at the heterointerfacealong the (0001) plane and (0001), respectively, with the polarizationfield inducing the 2DEG 211A adjacent to a top surface of the nanowire210A.

In either crystal orientation (FIG. 2A or FIG. 2D), both back barrierand charge inducing layer are gated by the gate stack 250. FIG. 2C is aband diagram of a non-planar group III-nitride transistor formed fromthe non-planar crystalline semiconductor body illustrated in FIG. 2B,with the wrap around gate structure as illustrated in FIG. 1A, inaccordance with the exemplary embodiment where the nanowire 210A (andnanowire 210B) is GaN and the semiconductor layer 215 is AlN. As shownin FIG. 2C, at zero gate bias the bands are asymmetrical across thecross-sectional width of the nanowire 210A and between the semiconductorlayer portion 215A and the semiconductor layer portion 215B. At thelocation within the nanowire 210A where the band bends below the Fermilevel E_(F) from the polarization field, charge carriers are present. Asillustrated by the band diagram, the symmetrical gate structureillustrated in FIG. 1B has an asymmetrical function arising from thepolarity of the nanowire 210A. The gate conductor 250 proximate to thesemiconductor layer portion 215B therefore modulates the charge carrierdensity of the 2DEG 111 within the channel region while the gateconductor 250 proximate to the semiconductor layer portion 215Amodulates the back barrier. For the orientation in FIG. 2D, the samegated back barrier is provided where the gate conductor fills betweenthe nanowire 210A and the substrate layer 205 to coaxially wrapcompletely around the nanowire 210A, as illustrated in FIG. 1A by thedashed lines within the channel region 245. The band diagram of FIG. 2Cis therefore also applicable to the crystal orientation of FIG. 2D.

As further shown in FIG. 1B, the gate stack includes a gate conductor250 electrically isolated from the nanowire 210A by a gate dielectricmaterial 240 disposed under the gate conductor to reduce leakage currentbetween the gate conductor and the nanowire 210A. As the gate stack 250Ais disposed over the semiconductor layer 215, the semiconductor layer215 and the gate dielectric material of the gate stack 215 may beconsidered a composite dielectric stack. In the embodiment illustratedin FIG. 1B, the gate dielectric material 240 is further disposed betweenthe substrate layer 205 and the gate conductor 250. As shown in FIG. 1A,coaxially wrapping entails the gate conductor within the region 250Adisposed between the nanowire 210A and the substrate layer 205.

Generally, the gate dielectric material 240 may include one or more ofany material known in the art suitable for FET gate dielectrics, and ispreferably a high K dielectric (i.e., having a dielectric constantgreater than that of silicon nitride (Si₃N₄)), such as, but not limitedto, high K oxides like gadolinium oxide (Gd₂O₃), hafnium oxide (HfO₂),high K silicates such as HfSiO, TaSiO, AlSiO, and high K nitrides suchas HfON. Similarly, the gate conductor 250 may be of any material knownin the art for transistor gate electrodes. In an embodiment, the gateconductor 250 includes a work function metal which may be selected toobtain a desired threshold voltage (V₁) (e.g., greater than 0V, etc).Exemplary conductive gate materials include, tungsten (W), aluminum(Al), titanium (Ti), tantalum (Ta), nickel (Ni), molybdenum (Mo),germanium (Ge), platinum (Pt), gold (Au), ruthenium (Ru), palladium(Pd), iridium (Ir), their alloys and silicides, carbides, nitrides,phosphides, and carbonitrides thereof.

Returning to FIG. 1A, the nanowire 210A further includes a source region220 and drain region 230. In the exemplary embodiment, within the sourceregion 220 and drain region 230, the nanowire 210A is the same groupIII-nitride semiconductor material present in the channel region 245,but may further include a higher concentration of dopant, such as ann-type impurity (i.e., N+). In certain embodiments, the nanowire 210Awithin the source region 220 and drain region 230 maintains the samemonocrystallinity as within the channel region 245. In a firstembodiment, a semiconductor material layer 212A is in contact with thenanowire 210 within the source and/or drain regions 220, 230. For thegroup III-nitride transistor 200, the nanowire 210A is sandwichedbetween semiconductor material layers 212A and 212B. In the exemplaryembodiment, where the semiconductor material layer 212A facilitates anundercut of the nanowire 210A within the channel region 245, thesemiconductor material layer 212A is of a different material than thenanowire 210A.

In embodiments, the semiconductor material layer 212A is any of thegroup III-nitrides described for embodiments of the nanowire 210A thatis other than the one utilized for the nanowire 210A. For example, thesemiconductor material layer 212A may be AlN, GaN, InN, Al_(z)In_(1-z)N,or Al_(z)Ga_(1-z)N, where z is different than x. In one embodiment, thesemiconductor material layer 212A has a lower bandgap than that of thenanowire 210A (e.g., z less than x where both the nanowire 210A andsemiconductor material layer 212A is ternary) to help reducesource/drain contact resistance. In an alternate embodiment, thesemiconductor material layer 212A has a higher bandgap than that of thenanowire 210A (e.g., z greater than x where both the nanowire 210A andsemiconductor material layer 212A is ternary). Where the groupIII-nitride of nanowire 210A and the group III-nitride of thesemiconductor material layer 212A is alloyed or intermixed, the bandgapwithin extrinsic drain region 235A may be advantageously increased to beintermediate of the bandgap in the channel region 245 and the bandgap ofthe semiconductor material layer 212A, enabling an even higher BV. Assuch, depending on the embodiment, the transistor 100 may incorporate asemiconductor that is sacrificial within the channel region 245 toselectively decrease or increase the bandgap of the semiconductormaterial employed in the channel region 245.

FIG. 1C is an isometric illustration of a group III-N transistor 200, inaccordance with an embodiment. While the group III-N transistor 100 hasan extrinsic drain region 235A that is approximately equal to thespacing between a source contact 222A and the channel region 245 (i.e.,determined by the dielectric spacer 255) the group III-N transistor 200has an extrinsic drain region 235B that is greater than the spacingbetween the source contact 222B and the channel region 245. Theextrinsic drain regions 235A, 235B may be lightly doped and furtherinclude the semiconductor layer 215, as illustrated in both FIGS. 1A and1C. Within the extrinsic drain regions 235A, B the semiconductor layer215 functions as a charge inducing layer and because the longitudinallength of the extrinsic drain region is a function the BV desired, thegroup III-N transistor 200 has a greater BV than the group III-nitridetransistor 100.

In one exemplary embodiment, the extrinsic drain region 235B is an alloyof the first and second semiconductor materials to provide a bandgapintermediate between to those of the first and second semiconductormaterials. As illustrated by the open arrows in FIG. 1C, within theextrinsic drain region 235B, the alloy of the first and secondsemiconductor materials is in the form of an a disordered multilayerstructure. The multilayer structure includes a group III-nitridematerial of the nanowire 210A, extending through the extrinsic drainregion 235B and coupling the channel region 245 to a drain region 230,sandwiched between semiconductor material layers 212A and 212B. Withopposing sides of the nanowire 210A adjacent to the semiconductormaterial layers 212A and 212B, good alloy uniformity may be achievedwithin the portion of the extrinsic drain region 235B.

In an embodiment a group III-nitride transistor includes a source regionand/or drain region of a nanowire embedded with a source and/or draincontact. FIG. 1C illustrates a source contact 222B that coaxially wrapscompletely around the nanowire 210A to fill in the gap between thenanowire 210A and the substrate layer 205. As shown in FIG. 1A, thesource contact 222A does not completely wrap around the nanowire 210Abecause the semiconductor layers 212A and 212B remain. The sourcecontacts 222A, 222B may includes an ohmic metallization layer and mayfurther include an epitaxially grown semiconductor of differentcomposition than the nanowire 210A. For example, the source contacts222A, 222B may include a tunnel junction (e.g., a p+ layer wrappingaround the nanowire 210A within the source region 220). Ultra steep turnon and off (i.e., improved subthreshold performance) may be providedwith such a tunnel junction for reduced off state leakage current.

The nanowire 210A further includes a drain region 230. Like the sourceregion, the drain region may be more or less embedded within a draincontact 232. In FIG. 1C, a drain contact 232B coaxially wraps completelyaround the nanowire 210A within the drain region 230 to fill in the gapbetween the nanowire 210A and the substrate layer 205. As shown in FIG.1A, the drain contact 232A does not completely wrap around the nanowire210A because the semiconductor layers 212A and 212B remain. Like thesource contacts 222A, 222B, the drain contacts 232A, 232B may include anohmic metallization layer and may also further include an epitaxiallygrown semiconductor of different composition than the nanowire 210A.

In embodiments, as shown in FIGS. 1A and 1C, a group III-N transistorincludes a vertical stack of nanowires to achieve a greater currentcarrying capability (e.g., larger drive currents) for a given footprintover a substrate layer. Any number of nanowires may be verticallystacked, depending on fabrication limitations, with the longitudinalaxis of each of the nanowire substantially parallel to a top surface ofthe substrate layer 205. In the exemplary embodiment illustrated in FIG.1A or 1C, each of the nanowires 210A, 210B is of the same firstsemiconductor material within the channel region 245. In furtherembodiments, each of the nanowires 210A and 210B is coaxially wrapped bythe gate stack 250A (e.g., as further shown in FIGS. 1B, 2B, and 2E). Atleast the gate dielectric layer 240 is to be disposed between thenanowires 210A and 210B, but in the exemplary embodiment shown in FIG.1B, the gate conductor is also present between the channel regions ofeach of the nanowires 210A, 210B.

In the embodiment illustrated in FIG. 1C, each of the plurality ofnanowires 210B are physically coupled together by semiconductor materialin the extrinsic drain region 235B. For embodiments including aplurality of nanowires 210A, 210B, the group III-N transistors 100, 200have a plurality of drain regions, one for each nanowire within thevertical stack of nanowires. Each drain region may be coaxially wrappedwith the drain contact (e.g., 232B) coaxially wrapping completely aroundeach of the drain regions filling in the gaps between the nanowires210A, 210B. Similarly, the source contact 222B may coaxially wrapcomplete around a plurality of source regions.

A brief description of salient portions of the fabrication process foreach of the transistors 200 and 201 is now provided. FIG. 3 is a flowdiagram illustrating a method 300 of fabricating the non-planar highvoltage transistors 200 and 201, in accordance with an embodiment. Whilemethod 300 highlights the main operations, each operation may entailmany more process sequences, and no order is implied by the numbering ofthe operations or relative positioning of the operations in FIG. 3.FIGS. 4A, 4B, 4C, 4D and 4E are isometric illustrations of thenon-planar group III-N transistors 100, 200 fabricated in accordancewith an embodiment of the method 300.

At operation 301, a stack of monocrystalline semiconductor materials isgrown using any standard chemical vapor deposition (CVD), molecular beamepitaxy (MBE), hydride vapor phase epitaxy (HVPE) growth technique,etc., (with standard precursors, temperatures, etc.). At least twodifferent semiconductor layers are grown as part of the epitaxial stack.In an embodiment, the layers 212A, 212B, and 212C are a first groupIII-nitride material with a second group III-nitride disposed betweenthem.

At operation 305, a nanowire is defined (e.g., length and width) byetching the epitaxial stack with any plasma or wet chemical etchtechnique known in the art for the particular materials grown as part ofthe epitaxial stack. As shown in FIG. 4A, at operation 303 the finstructure 410 is etched into an epitaxial stack of to form nanowires210A, 210B alternating with semiconductor layers 212A, 212B, and 212C.As shown each of the nanowires 210A and 210B are disposed both above andbelow the semiconductor layers 212A, 212B. The layer thicknesses, T₁-T₄are dependent on the desired nanowire dimensions and also on the abilityto backfill the thicknesses T₁, T₃ with the gate stack after thesemiconductor layer 215 is formed on the nanowires 210A and 210B. Alsoshown in FIG. 4A, an insulator layer 407 is formed on either side of thefin structure 410 over the substrate layer 205, for example by a shallowtrench isolation technique.

Returning to FIG. 3, at operation 305, a drain contact is formed to wraparound the nanowire 210A and 210B either partially (as in FIG. 1A) orcompletely (as in FIG. 1C). At operation 310, a source contact issimilarly formed. At operation 315 a gate conductor is coaxially wrappedcompletely around the nanowires 210A and 210B longitudinal channellength of. The device is then completed at operation 320, for exampleusing conventional interconnect techniques.

FIG. 4B illustrates one embodiment of the operations 305, 310 and 315entailing formation of a sacrificial gate 412 disposed on the finstructure 410. In one such embodiment, the sacrificial gate 412 iscomposed of a sacrificial gate oxide layer and a sacrificial polysilicongate layer which are blanket deposited and patterned with a conventionallithographic and plasma etch process. Spacers may be formed on thesidewalls of the sacrificial gate 412 and an interlayer dielectric layermay be formed to cover the sacrificial gate 412. The interlayerdielectric layer may be polished to expose the sacrificial gate 412 fora replacement gate, or gate-last, process. Referring to FIG. 4C, thesacrificial gate 412 has been removed, leaving spacers 255 and portionof the interlayer dielectric layer (ILD) 420, 421. As further shown inFIG. 4C, the semiconductor layers 212A, 212B, and 212C are removed inthe channel region originally covered by the sacrificial gate 412.Discrete nanowires 210A and 210B of the first semiconductor materialthen remain.

As shown in FIG. 4D, the gate stack 250A is then formed coaxiallywrapping around the nanowires 210A, 210B within the channel region 245.FIG. 4D illustrates the gate stack following epitaxial growth of thesemiconductor layer 215, deposition of the gate dielectric 240, anddeposition of the gate conductor 250 to backfill the gap formed byselectively etching the semiconductor layers 212A, 212B, and 212C. Thatis, the gate stack is formed in the trench between the interlayerdielectric layers 420, 421 after etching the epitaxial stack intodiscrete group III-N nanowires. Additionally, FIG. 4D depicts the resultof the subsequent removal of the interlayer dielectric layer 420 afterformation of the gate stack 250A. For embodiments including theextrinsic drain region 235B (e.g., group III-nitride transistor 200), aportion of the interlayer dielectric layer 421 is retained (e.g., with alithographically defined masked etch of the interlayer dielectric)within the extrinsic drain region 235A. For alternate embodiments (e.g.,for transistor group III-nitride transistor 100) no portion of the ILD421 is retained.

For embodiments which include source and drain contacts coaxiallywrapping completely around the nanowires 210A, 210B, once the ILD layers420, 421 are removed, the portions of the semiconductor layers 212A and212B not protected by the gate stack (and any portion of the interlayerdielectric layer 421 remaining) are then removed selectively relative tothe nanowires 210A, 210B of the first semiconductor material to form agap between the nanowires 210A, 210B and the substrate layer 205.Discrete portions of the first semiconductor then remain in the sourceand drain regions 220 and 230, as depicted in FIG. 4D. The source anddrain contacts 222, and 232 (as illustrated in FIG. 2A) may then beformed by backfilling the gaps formed within in the source and drainregions 220, 230. In one such embodiment, a contact metal is conformallydeposited by CVD, atomic layer deposition (ALD), or metal reflow. Inalternative embodiments, source and drain contacts 222A, 232A are formedon the semiconductor layers 212A and 212B as well as sidewalls of thenanowires 210A and 210B within the source and drain regions. The devicesare then ready to be completed at operation 320, for example withconventional interconnect metallization, etc.

In a further embodiment, illustrated by FIG. 4E, any retained portion ofthe ILD 421 may removed selectively to the spacers 255, gate conductor250, and source, drain contacts 222B, 232B. One of the nanowires 210A,210B and the semiconductor layers 212A, 212B, 212C may then be removedselectively over the other of the nanowires 210A, 210B and thesemiconductor layers 212A, 212B, 212C. In the exemplary embodiment, acrystalline semiconductor material that has a larger bandgap than thatof the nanowires 210A, 210B is then epitaxially regrown in the undercutgap(s). Semiconductor layers 212A, 212B, 212C may then function ascharge inducing layers within the extrinsic drain region 235B.Semiconductor layer 215 may additionally be grown on sidewalls of thenanowires 210A, 210B (e.g., where the crystal orientation is as in FIG.2B). Alternatively, or in addition, a diffusive element, such as Al, Ga,or Zn may be incorporated on the first semiconductor within theextrinsic drain region 235B after the remaining portion of the ILD 421is removed.

In embodiments, the semiconductor materials present within the extrinsicdrain region 235B are alloyed by a thermal annealing. For example, thefirst semiconductor material 210A, 210B and semiconductor layers 212A,212B, and 212C may be intermixed (i.e., multilayered structuredisordering). Alternatively, the thermal annealing may intermix thesemiconductor materials with the diffusive element (e.g., Al, Ga, orZn). In one such embodiment, the thermal anneal is simultaneous with theformation of the source and drain contacts (e.g., operations 305 and310). Notably, replacement of the sacrificial gate 412 may occur afterformation of the source and drain contacts 222, 232 and/or afterannealing of the extrinsic drain region 235B. Also, the semiconductorlayer 215 may be grown after thermal annealing of the extrinsic drainregion 235B to maintain an abrupt heterointerface between thesemiconductor layer 215 and the nanowires 210A, 210B within the channelregion 245 and/or extrinsic drain region 235B.

FIG. 5 is a functional block diagram of a SoC implementation of a mobilecomputing platform, in accordance with an embodiment of the presentinvention. The mobile computing platform 700 may be any portable deviceconfigured for each of electronic data display, electronic dataprocessing, and wireless electronic data transmission. For example,mobile computing platform 700 may be any of a tablet, a smart phone,laptop computer, etc. and includes a display screen 705 that is in theexemplary embodiment a touchscreen (e.g., capacitive, inductive,resistive, etc.) permitting the receipt of user input, the SoC 710, anda battery 713. As illustrated, the greater the level of integration ofthe SoC 710, the more of the form factor within the mobile computingdevice 700 that may be occupied by the battery 713 for longest operativelifetimes between charging, or occupied by memory (not depicted), suchas a solid state drive for greatest functionality.

Depending on its applications, mobile computing platform 700 may includeother components including, but are not limited to, volatile memory(e.g., DRAM), non-volatile memory (e.g., ROM), flash memory, a graphicsprocessor, a digital signal processor, a crypto processor, a chipset, anantenna, a display, a touchscreen display, a touchscreen controller, abattery, an audio codec, a video codec, a power amplifier, a globalpositioning system (GPS) device, a compass, an accelerometer, agyroscope, a speaker, a camera, and a mass storage device (such as harddisk drive, compact disk (CD), digital versatile disk (DVD), and soforth).

The SoC 710 is further illustrated in the expanded view 720. Dependingon the embodiment, the SoC 710 includes a portion of a silicon substrate500 (i.e., a chip) upon which two or more of a power managementintegrated circuit (PMIC) 715, RF integrated circuit (RFIC) 725including an RF transmitter and/or receiver, a controller thereof 711,and one or more central processor core 730, 731 is fabricated. As willbe appreciated by one of skill in the art, of these functionallydistinct circuit modules, CMOS transistors are typically employedexclusively except in the PMIC 715 and RFIC 725, which typically utilizeLDMOS and III-V HBT technologies, respectively. In embodiments of thepresent invention however, the PMIC 715 and RFIC 725 employ the groupIII-nitride transistors described herein (e.g., group III-nitridetransistor 100 or 200). In further embodiments the PMIC 715 and RFIC 725employing the group III-nitride transistors described herein areintegrated with one or more of the controller 711 and processor cores720, 730 provided in silicon CMOS technology monolithically integratedwith the PMIC 715 and/or RFIC 725 onto the silicon substrate 500. Itwill be appreciated that within the PMIC 715 and/or RFIC 725, the highvoltage, high frequency capable group III-nitride transistors describedherein need not be utilized in exclusion to CMOS, but rather siliconCMOS may be further included in each of the PMIC 715 and RFIC 725

The RFIC 725 may implement any of a number of wireless standards orprotocols, including but not limited to Wi-Fi (IEEE 802.11 family),WiMAX (IEEE 802.16 family), IEEE 802.20, long term evolution (LTE),Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT,Bluetooth, derivatives thereof, as well as any other wireless protocolsthat are designated as 3G, 4G, 5G, and beyond. The platform 725 mayinclude a plurality of communication chips. For instance, a firstcommunication chip may be dedicated to shorter range wirelesscommunications such as Wi-Fi and Bluetooth and a second communicationchip may be dedicated to longer range wireless communications such asGPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, and others.

The group III-nitride transistors described herein may be specificallyutilized where a high voltage swings present (e.g., 7-10V battery powerregulation, DC-to-DC conversion, etc. within the PMIC 715). Asillustrated, in the exemplary embodiment the PMIC 715 has an inputcoupled to the battery 713 and has an output provide a current supply toall the other functional modules in the SoC 710. In a furtherembodiment, where additional ICs are provided within the mobilecomputing platform 700 but off the SoC 710, the PMIC 715 output furtherprovides a current supply to all these additional ICs off the SoC 710.As further illustrated, in the exemplary embodiment the RFIC 715 has anoutput coupled to an antenna and may further have an input coupled to acommunication modules on the SoC 710, such as an RF analog and digitalbaseband module (not depicted). Alternatively, such communicationmodules may be provided on an IC off-chip from the SoC 710 and coupledinto the SoC 710 for transmission. Depending on the group III-nitridematerials utilized, the group III-nitride transistors described herein(e.g., 200 or 201) may further provide the large power added efficiency(PAE) needed from a power amplifier transistor having an F_(t) of atleast ten times carrier frequency (e.g., a 1.9 GHz in an RFIC 725designed for 3G or GSM cellular communication).

It is to be understood that the above description is illustrative, andnot restrictive. For example, while flow diagrams in the figures show aparticular order of operations performed by certain embodiments of theinvention, it should be understood that such order may not be required(e.g., alternative embodiments may perform the operations in a differentorder, combine certain operations, overlap certain operations, etc.).Furthermore, many other embodiments will be apparent to those of skillin the art upon reading and understanding the above description.Although the present invention has been described with reference tospecific exemplary embodiments, it will be recognized that the inventionis not limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The scope of the invention should, therefore, be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A group III-N transistor, comprising: a nanowiredisposed on a substrate, wherein a longitudinal length of the nanowirefurther comprises: a channel region of a first group III-N material, thefirst group III-N material having the crystal structure of wurtzitestructure; a source region electrically coupled with a first end of thechannel region; a drain region electrically coupled with a second end ofthe channel region; and an extrinsic drain region comprising a secondgroup III-N material separating the drain region from the channelregion, and a gate stack coaxially wrapping completely around thechannel region.
 2. The group III-N transistor of claim 1, wherein thesecond group III-N material has a wider bandgap than that of the firstgroup III-N material.
 3. The group III-N transistor of claim 1, whereinthe extrinsic drain region is an alloy of the first and second groupIII-N materials with a bandgap intermediate between those of the firstand second group III-N materials.
 4. The group III-N transistor of claim1, wherein the first group III-N material consists essentially of GaN,or consists essentially of InN, or consists essentially ofAl_(x)In_(1-x)N, where x is less than 1, or consists essentially ofAl_(x)Ga_(1-x)N, where x is less than
 1. 5. The group III-N transistorof claim 4, wherein the second group III-N comprises AlN, GaN, InN,Al_(z)In_(1-z)N, or Al_(z)Ga_(1-z)N, where z is different than x.
 6. Asystem on chip (SoC), comprising: a power management integrated circuit(PMIC) including at least one of a switching voltage regulator orswitching mode DC-DC converter; and an RF integrated circuit (RFIC)including a power amplifier operable to operate with a cut-offfrequency, F_(t) and maximum oscillation frequency, F_(max) of both atleast 20 GHz, and generate a carrier wave frequency of at least 2 GHz,wherein both of the PMIC and RFIC are monolithically integrated onto asame substrate, and wherein at least one of PMIC and RFIC include thegroup III-N transistor of claim
 1. 7. A mobile computing device,comprising: a touch screen; a battery; an antenna; and the SoC of claim6, wherein the PMIC is coupled to the battery and wherein the RFIC iscoupled to the antenna.